In the field of electric drive vehicles, electrical braking is a feature commonly utilized to slow the vehicle without applying the service brakes. This feature is accomplished by energizing the motor in a direction opposite to the immediate direction of rotation. For example, when a vehicle traveling in the forward direction is shifted into reverse, the control responds by energizing the motor at a constant excitation in the reverse direction. The constant excitation provides for a gradual nonlinear deceleration. As the speed of the motor armature decreases the constant excitation results in less armature current and a corresponding reduction in braking torque. Such nonlinear systems are adequate for operator controlled vehicles, as precise positioning via electrical braking is not required.
However, in the field of automatic guided vehicles (AGV) constant deceleration is an important tool in maintaining navigational accuracy. It is of paramount importance that the AGV be precisely positionable to allow the transported material to be deposited or collected at highly defined locations. The ease of predicting vehicle location during linear deceleration is apparent, owing to the single variable of time. Conversely, nonlinear deceleration unnecessarily complicates the navigational algorithms and while the possibility of constructing such a system exists, it is undesirable. Further, any simplifying assumptions to the nonlinear algorithm manifest in positional degradation.
Prior systems, such as U.S. Pat. No. 3,995,204 issued Nov. 30, 1976 to Konrad et al, have successfully closed the loop on armature current. By controlling armature current to a constant value during electrical braking the system can provide linear deceleration. Such a system suffers from the inherent disadvantages of additional circuitry. A closed loop current feedback system to a digital system necessarily requires a current sensor such as a calibrated resistor, amplifier circuitry, and an analog to digital converter. Increased cost as well as decreased reliability are obvious shortcomings to such an analog based system.
The present invention is directed to overcoming one or more of the problems as set forth above.